1. Field of the Invention
The present invention relates to a sliding device composed of a combined structure of a screw bolt and a nut, particularly to an optimal combination of a screw bolt and a nut used as a component of an actuator.
2. Description of the Related Art
Recently, cellular phones including a down-sized camera and/or high functional cameras (increased pixels, auto-focusing (AF) function and the like) are popularly used, and a zooming mechanism is installed in a down-sized camera. In order to move a lens in the zooming mechanism, a method using a rapid deformation (impact driving) of a piezoelectric element is employed. Also, a unit using the piezoelectric element is mounted on a digital camera as a vibration compensation device (type of directly moving CCD) for anti-shake system. Reasons why the piezoelectric element is used is that when an electromagnetic motor is used, for example, an electromagnetic field generated in a cellular phone may cause an electromagnetic noise to make the size reduction hard and the assembly difficult. Also, as a method of moving a lens in an auto-focusing mechanism, combination of a stepping motor and a gear or combination of a voice coil motor and a spring have been popularly used, but there are plenty of objects to lessen volume required for a mechanism and/or a power consumption and the like.
As a method that can solve those problems, applying of an auto-focusing system using a piezo motor has been increased. The term “piezo” means “to apply pressure” in Greek. When a pressure is applied to a crystal, an electric charge is generated in proportion. When the phenomenon is used in reverse, a volume is contracted by applying a voltage to a piezo element. The phenomenon is applied to make use for positioning of the piezo motor. The piezo motor has a property that the element is contracted with applied voltage, and the property is used to cause a vibration and transmit the vibration to another element. The property is used with skill for an actuator which convert received vibration of the piezo element to a driving force in the auto-focusing mechanism using the piezo motor for continuous movement of a lens for focusing.
By using the piezo motor, positioning of lens movement can be made precisely and moreover, complicated mechanical components are not required any more to result improved reliability and silence since the lens is driven only by voltage control to the element. Also, when the piezo motor is used, durability can be easily improved, problems hardly occurs even after repeating motion for several million times and quick response and high energy efficiency can be achieved. Which is an excellent characteristic as compared with conventional motor driving. However, because of the quick response, fine adjustment is difficult. So, a system adopting a combined structure of a lead screw and a stator for a piezoelectric element and lens driving is employed in the auto-focusing mechanism. When the structure of the lead screw having a male thread combined with the stator in which a female thread is formed in a female thread hole on an inner circumference is applied, a feature that a rotating motion of a screw is converted to a linear motion is used to enable fine adjustment.
However, a problem in such a case is damage caused by slight relative reciprocal sliding motion, that is, fretting motion. The damage occurs locally at a contact portion of members in a mechanical component subjected to vibration, repeated stress, fluctuating stress and the like. In the combined structure of the lead screw and the stator, surface damage called fretting wear may easily occur when two faces are slightly slide while in contact. With progress of the fretting wear, reddish-brown fine oxidized abrasion powder called cocoa takes place in the air at a contact portion in a steel materials. Taking place of the oxidized abrasion powder may cause back-lash to reduce mechanical accuracy and functions extremely, also which causes vibration and/or noise. Moreover, adhesion of the abrasion powder might make disassembly and/or repair impossible.
As described above, the phenomenon caused by fretting wear is serious surface damage in the fields of tribology and material strength. When the repeating stress is put on the contact portion at the same time, fretting fatigue occurs, and fatigue cracks might be generated in a damaged portion, which progresses and results destruction of the member and extremely reduces fatigue strength of the member.
In order to solve such problems, in Japanese Patent Application (National Publication of International Patent Application No. 2007-505599), a device for driving a shaft assembly comprising a shaft with screw having a rotating shaft and a nut screwed therewith, wherein
a) the assembly contains means for giving an ultrasonic vibration to the nut, by which the shaft with screw penetrating the nut is rotated and moved in the axial direction at the same time;
b) the shaft with screw is connected to a load that can move in the axial direction; and
c) the assembly is further including means for giving an axial force to the shaft with screw
is proposed. Also, in the embodiment, as a material for a shaft 12 with screw and a screwed nut 16, the combination employing stainless for the shaft 12 and brass for the nut 16 is preferred to minimize abrasion.
However, in Japanese Patent Application (National Publication of International Patent Application No. 2007-505599), as the materials of the shaft 12 with screw and the screwed nut 16 in the embodiment of the description, the shaft 12 with screw is made of substantially stainless steel, and a brass nut is used for the nut 16 to be engaged with the shaft, but with the combination of the shaft with screw and the screwed nut by the materials disclosed in Japanese Patent Application (National Publication of International Patent Application No. 2007-505599), it is actually difficult to obtain sufficient durability as a sliding device. Also, in the embodiment of the application, the thread shape of the shaft 12 with screw is preferably notched, and a pitch of the screw is preferably approximately 200 or less thread grooves per inch.
As mentioned above, those with more excellent durability have been required for a sliding device screwed capable of screw rotation and subjected to a sliding load during repeating screw rotation.